The route and rate of aerobic degradation of parent thiodicarb in the laboratory in the dark has only been determined in 3 soils at 20°C (± 2°C, soil moisture adjusted to 45% of maximum moisture holding capacity), instead of the four specified in SETAC guidelines. One of the three soils used in the 20°C study was also used in an aerobic laboratory study at 10°C (± 2°C, soil moisture adjusted to 45% of maximum moisture holding capacity). Another aerobic study was carried out in an additional soil at 25°C (± 1°C, 75% field moisture capacity at 1/3 bar) to EPA guidelines. The studies were run for up to 60 days. Unextractable residues accounted for up to 65.6% of the total at study end (56 days). CO2 accounted for up to 30.2% (56 days). Methomyl was the only major (= 10%AR) metabolite identified in the aerobic studies. The other metabolite identified was methomyl oxime. Thiodicarb was rapidly metabolised to methomyl under aerobic soil conditions. Methomyl formation increased rapidly and then gradually decreased. Methomyl was gradually metabolised to methomyl oxime which was rapidly metabolised to primarily carbon dioxide and a small amount of acetonitrile. Methomyl oxime did not accumulate in aerobic sandy loam soil. Thus carbon dioxide was the ultimate major degradation product of thiodicarb aerobic soil metabolism (and unextracted soil residues). Aerobic laboratory soil metabolism studies show a consistent pattern of degradation for thiodicarb. Thiodicarb is rapidly metabolised to methomyl, which then degrades to carbon dioxide through methomyl oxime. No thiodicarb-derived metabolites other than methomyl ever exceed 4% of the total applied parent. In the methomyl degradation studies, no major (>10% AR) metabolites were produced under aerobic nor under anaerobic conditions. Under aerobic conditions, unextractables accounted for up to 32.2% AR at study end (30 days) and CO2 accounted for up to 75.5% AR at study end (3 months).